1. Field of the Invention
The present invention relates: generally to press fabrics which are used in industrial filtration processes; more specifically to fabrics which are utilized in presses which apply roller nip pressure to the fabric and the product; and most particularly to fabrics which are used in high nip pressure presses which produce paper pulp as an intermediary product in the papermaking process.
In a typical process, water laden pulp is presented to the press for dewatering. The pulp slurry as presented to the press must be dewatered to increase the consistency of the slurry by about 20 to 25 times. Typically, the press is about 80 inches wide, however, units having a width of about 136 inches are known. The typical press has opposed fabrics mounted in an endless fashion about a series of rollers. Some of the rollers are opposed and form a roller nip which presses the fabrics and the product. Pressure nips of about 300 pli are known, however, lower nip pressures are more typical.
One proposed unit is expected to dewater a pulp slurry having a Ph between 6.0 and 7.0 and a minimum freeness of about 550 CSF (Canadian Standard Freeness). The slurry inlet consistency is expected to be about 1.5% solids and the outlet consistency is projected at about 35% solids. Projected operating speeds for the unit are up to 165 feet per minute. The unit is expected to have nip pressures approaching 460 pli. The unit fabric width is expected to exceed twice the typical width of about 80 inches. Thus, the proposed unit will utilize a fabric width and high nip pressures which exceed known units. In fact, nip pressures of over 400 pli are not believed to have been known in the prior art. The prior art fabrics are not suitable for pulp units having high nip pressures which exceeded 300 pli.
As a result of the above, it was recognized that the industry required a belt filter press fabric which was capable of maximizing service life, drainage, fiber retention and wear characteristics. Additionally, it was recognized that the preferred fabric should provide optimal performance with respect to fabric cleaning and sheet release properties.
In view of the above, it was recognized that the yarns comprising the belt filter press fabric had to be selected for certain wear characteristics and it needed to be configured in a structure which provided additional wear characteristics as a result of that construction.
2. Description of the Prior Art
In the prior art of papermaking fabrics, it has been recognized that papermakers fabrics may be made from combinations of yarns which impart separate characteristics to the fabric. One example of such a fabric is disclosed in U.S. Pat. No. 4,289,173. This patent discloses the use of separate yarns of different materials in order to obtain different properties, preferably improved wear resistance, and dimensional stability in a formation fabric. U.S. Pat. No. 4,289,173 is concerned with a papermakers forming fabric and does not relate to the problems associated with press fabrics that are subjected to the high nip pressure associated with the present invention. As will be known to those skilled in the art, the formation process, except in the case of twin wire formation, is generally an open process which does not employ opposed belts or nip pressures. The twin wire process does not utilize nip rollers and the only pressure is that resulting from the positioning of the opposed belts relative to each other.